Excitatory amino acids such as glutamic acid have been shown to be important neurotransmitters (Johnson, R. L.; Koerner, J. F., J. Med. Chem. 1988, 31, 2057), which in excess participate in the sequence of events leading to neuronal damage after cerebral ischemia (Choi, D. W., Trends Neurosci. 1988, 11, 465). One important subtype of excitatory amino acid receptor is the NMDA-receptor, which is defined by the selective agonist N-methyl-D-aspartic acid (NMDA). Blocking the action of endogenous agonist by the selective NMDA-receptor antagonist 4-(3-phosphonopropyl)-2-piperazinecarboxylic acid (CPP) has been shown to prevent ischemic brain damage in gerbils (Boast, C. A. et al., Brain Research, 1988, 442, 345). Also, NMDA-induced convulsions have been prevented by CPP in mice (Lehmann, J. et al., J. Pharmacol. Exp. Ther. 1987, 240, 737). Finally, competitive NMDA antagonists such as CPP have been shown to prevent the Parkinsonian-like symptoms induced by MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) in rats (Turski, L. et al., Nature 1991, 349, 414). For these reasons, NMDA-receptor antagonists are considered appropriate for treatment of epilepsy, stroke (Engelsen, B., Acta. Neurol Scand. 1986, 74, 337), and neurodegenerative disorders such as Alzheimer's disease (Maragos, W. F. et al., Trends Neurosci. 1987, 10, 65) and Parkinson's disease.
Chemical entities known to be competitive NMDA-receptor antagonists contain the .alpha.-amino-carboxylic acid and phosphonic acid functionalities separated by a variety of spacer units. An unembellished example is 2-amino-5-phosphonovaleric acid (AP5) (Watkins, J. C.; Evans, R. H., Annu. Rev. Pharmacol. Toxicol. 1981, 21, 165), which contains a saturated carbon chain. More complex examples, which contain elements enhancing structural rigidity and therefore potency, include CPP (see above), cis-4-(phosphonomethyl)-2-piperidinecarboxylic acid (CGS-19755) (Lehman, J. et al., J. Pharmacol. Exp. Ther. 1988, 246, 65), and (E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid (CGP-37849) (Schmutz, M. et al., Abs. Soc. Neurosci. 1988, 14, 864). Although there has been effort to find groups which are bioisosteric with the phosphonic acid group (Chenard, B. L. et al., J. Med Chem. 1990, 33, 1077), no examples of NMDS-receptor antagonists have appeared in the literature which demonstrate a bioisosteric replacement of the .alpha.-aminocarboxylic acid functionality.